This is why you don't trust source code in a matter like this. You disassemble and/or decompile the code on the machines themselves and reverse engineer it. Once you understand the system calls of the OS being used and are familiar with the fundamentals of the CPU architecture, it just takes a little time.

A way to check the source code is you take two machines, one suspect and another a control, compile the code they give you (assuming you can get it with a court Order) and put it on the control. Then you run several stacks of the same ballots through both machines and see if they give the same results. If not, they didn't give you the real source code.